Fluids drilling

Earlier on when we described the cutting action of the drill bit we learned about the drilling fluid or mud. The mud cools the bit and also removes the cuttings by carrying them up the hole outside the drill pipe. At the surface the mud runs over a number of moving screens, the shale shakers (Fig. 3.11) which remove the cutting for disposal. The fine particles which pass through the screens are then removed by desanders and desilters, usually hydrocyclones. 

Originally, mud was made from clay mixed with water, a simple system. Today the preparation and treatment of drilling fluid has reached a sophistication which requires specialist knowledge. The reason for this becomes clear if we consider the properties expected. 

The choice of drilling fluid has a major impact on the evaluation and" production of a well. Later in this section, we will investigate the interaction between drilling fluids, logging operations and the potential damage to well productivity caused by mud invasion into the formation. 

A mud motor (Fig. 3.17) is a positive displacement hydraulic motor, driven by the circulated drilling fluid. A continuous seal is formed between the body ( stator ) and the... 

Collapse load originates from the hydrostatic pressure of drilling fluid, cement slurry outside the casing and later on by moving formations e.g. salt... 

Sometimes primary cementations are not successful, for instance if the cement volume has been wrongly calculated, if cement is lost into the formation or if the cement has been contaminated with drilling fluids. In this case a remedial or secondary cementation is required. This may necessitate the perforation of the casing a given depth and the pumping of cement through the perforations. 

In some cases when drilling fluids invade a very low permeability zone, pressure equalisation in the formation can take a considerable time. The pressure recorded by the tool will then be close to the pressure of the mud and much higher than the true formation pressure. This is known as supercharging. Supercharging pressures indicate tight formation, but are not useful in establishing the true fluid pressure gradient. 

A traditional abandonment process begins with a well killing operation in which produced fluids are circulated out of the well, or pushed ( bull headed ) into the formation, and replaced by drilling fluids heavy enough to contain any open formation pressures. Once... 

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